1. Field of the Invention
This invention pertains to apparatus in the ditch digging field, and more particularly, to the large, self-powered ditchers having an engine which supplies power not only to the cutting wheel but also to the wheels or tracks of the ditcher. In this type ditcher, the engine cannot stop without severe damage to downstream components when the cutting wheel is suddenly stopped, thereby creating the need for some means to protect the engine and linkage when the cutting wheel is unexpectedly subjected to a significantly increased load, or stopped altogether.
2.Prior Art
The large, vehicular ditchers with which this invention is used are typically characterized by a tractor-like main frame to which a boom assembly and cutting wheel are attached. The engine in the ditcher produces the power not only to move the ditcher forward, but also to rotate the cutting wheel. These devices can be immense, with cutting wheels reaching 10 ft. in diameter. In operation, the cutting wheel is rotated and lowered into the ground, excavating dirt until the wheel has cut to the desired depth of the ditch. Then the ditcher is placed in forward motion and the ditch is extended.
In operation, it is not unusual for the cutting wheel to encounter an unforeseen obstruction which it cannot dig through at the forward speed at which the ditcher is travelling at the moment of encounter. When this happens, the cutting wheel may be stopped instantaneously. At that instant, the large engine in the ditcher which is providing the power to the cutting wheel is engaged. Accordingly, to avoid tearing the equipment apart, these ditchers necessarily employ some means for allowing slippage between the engine and the cutting wheel. Several alternative means have been employed. For example, a belt drive connection has been tried, although these are not suitable with the large ditchers because adequate tensioning cannot be obtained, such that too much slippage occurs.
Another device previously utilized in large ditchers for this purpose has been an over-centered clutch assembly. In this device, two or more dry clutch plates act upon one another frictionally to transfer power from the engine to the cutting wheel. The clutch plates are brought together by means of an overcentered toggle arrangement which includes a fine thread adjusting collar for increasing or decreasing the amount of friction between the plates. In operation, because of the coefficient of friction between the two plates, rotation of one plate is transferred to the other plate such that power from the engine is communicated to the cutting wheel. Then, should the cutting wheel strike an unforeseen obstruction or become subject to an excessive load, the torque thereby created overcomes the coefficient of friction between the two plates such that they begin to move relative to one another.
Although this device does work, it is subject to two substantial drawbacks. First, the coefficient of friction between the clutch plates is higher in the dynamic state than in the static state. Accordingly, as the plates first begin to move relative to one another, the coefficient of friction between them increases, actually working against a desired goal of of the device, which is to slip once the torque load exceeds a damageproducing level. The second major drawback is that these clutch plates, when they move relative to one another, generate a tremendous amount of heat very quickly. That heat causes the plates to expand, thereby increasing the coefficient of friction, compounding the situation and working against the desired goal. The heat build-up in this instance is such that the adjustable collar must be reset to compensate, then reset again when the plates have cooled.
Accordingly, there existed a need in the art for an improved means for limiting the torque which could be applied by the engine to downstream components.